The present invention relates in general to an improved method for forming products from a foamable dielectric composition, wherein the composition comprises a melt extrudable perfluorocarbon resin containing therein a polytetrafluoroethylene (PTFE) nucleating agent for controlling the size of the cellular structure of the foam, yet which does not significantly degrade the strength of the resin and, consequently, which permits blowing the resin to a density as low as 0.50 g./cc. with a suitable volatile fluid such, for example, as a lower fluorocarbon preferably having one or two carbon atoms--i.e., a fluoromethane represented by the formula: ##STR1## or, preferably, a fluoroethane represented by the formula: ##STR2## where X is selected from the group consisting of fluorine, chlorine, bromine and hydrogen. The present invention is particularly advantageous for use in the cable industry where, for example, the inventive composition and process can be used to form a foamed fluorinated ethylene-propylene (FEP) polymer--bonded to one or more conductors; and, where the foamed polymer can comprise either an insulating jacket surrounding one or more conductors and/or a dielectric material used to hold the inner and outer conductors of a coaxial cable in the desired spaced relationship.
Coaxial cables with a foam dielectric between the inner and outer conductors have been in commercial production since at least the 1950's. The dielectric loss of such cables has always been higher than that of the so-called "air dielectric" cables which use solid dielectric elements such as beads, helixes, or the like to hold the inner and outer conductors in the desired spaced relation; but, the disadvantage of higher dielectric loss of the foamed dielectric has been offset by the advantage of the foam in blocking the transmission of moisture into and through the cable, thereby eliminating the need for gas pressurization or evacuation systems to keep moisture out of the cable. Moisture, of course, greatly increases the losses in coaxial cables and may, in fact, render such cables inoperative.
Over the years since the introduction of foam dielectric cables, a number of different dielectric compositions have been used and/or proposed for use. Additionally, a number of different techniques have either been used or proposed for use: (i) to foam the dielectric resin; (ii) to apply the dielectric resin to the cable; (iii) to control the size, uniformity and structure of the cells in the foam; and (iv), to treat the foam after it has been formed. For example, a number of different dielectric materials and blowing agents or gas sources have been used or proposed for use in the manufacture of such cables. The foaming of the dielectric plastic resin has generally been effected either by the incorporation of a chemical blowing agent in the molten resin which is then thermally decomposed, or by the injection of a volatile fluid directly into the molten resin during extrusion thereof. The direct injection technique makes it more difficult to control the density and cell size of the foam, but produces a lower loss foam without the necessity of a drying step to remove moisture, which is one of the reaction products produced by some of the chemical blowing agents. The present assignee's Canadian Pat. No. 931,719 discloses a process which used a combination of both of the foregoing foaming techniques. Still another method involves swelling the resin in a suitable solvent and, thereafter, extruding the swollen resin at a temperature well above the boiling point of the solvent.
Certain of the foam jacketed and/or dielectric cables heretofore made have had the foam adhesively bonded to the conductor to more firmly "lock" the foam and conductor together and/or to insure blockage of fluid flow along the interface between the foam and the conductor. Bonding of the foam to the conductor has also been effected by heating the conductor. Other foam dielectric coaxial cables have been made without any bond between the foam and the conductors, still achieving uniform spacing between the inner and outer conductors and relatively tight engagement of the foam with the inner conductor. It is believed that any and all of the foregoing techniques may be used with compositions and processes embodying the present invention.
The melt extrudable resin used in the present invention is a perfluorocarbon resin, copolymers of tetrafluoroethylene and hexafluoropropylene. Such perfluorocarbon materials are commonly referred to as "fluorinated ethylene-propylene" (FEP) polymers. In the aforesaid Randa U.S. Pat. No. 3,072,583 it has been recognized that FEP polymers may be easily fabricated and possess excellent properties in terms of dielectric strength and high melting point, thereby making such materials particularly suitable for use as a foamed dielectric in, e.g., coaxial cables or the like.
It has, therefore, been proposed in the aforesaid Randa U.S. Pat. No. 3,072,583 that a melt extrudable FEP resin be foamed using a fluoromethane--preferably, either dichlorodifluoromethane or chlorodifluoromethane--blowing agent and boron nitride as a nucleating agent. However, I have found that the use of boron nitride as a nucleating agent imposes severe undesired constraints on both the foaming process and the characteristics of the resulting foamed product; apparently because if sufficient boron nitride is added to the FEP resin to produce a small cell structure--e.g., cells on the order of 1 to 40 mils. in diameter--the melt strength of the resinous material is significantly decreased, thereby precluding blowing the resin to densities as low as, e.g., 0.5 g./cc. Rather, with small cellular structure on the order of 1 to 40 mils., it appears that the densities normally achievable are on the order of from 0.93 g./cc. to 1.5 g./cc. Conversely, if the amount of boron nitride added to the FEP resin is reduced so as to maintain relatively high strengths for the resinous material, thereby permitting blowing of the material to low density, the cell structure is degraded and cell size becomes objectionably large. It is believed that the foregoing problem--viz., an inability to obtain both (i) uniform small sized cell structure and (ii) low density--is, at least to a degree, further compounded when using fluromethane blowing agents.